Power is difficult to produce at millimeter wave frequencies due to the low power output of transistors and the losses incurred by traditional power combiners at these frequencies. Free space combining, also called “quasi-optical” combining, eliminates the latter problem by allowing electromagnetic energy to combine in free space. Quasi-optical arrays can provide high power by combining the outputs of many (e.g. thousands) of elements.
Quasi-optical amplifiers arranged in arrays have been developed by a number of groups to produce high output powers at millimeter wave frequencies. These amplifier arrays amplify incoming radiation, either through reflection or transmission, and reradiate energy typically in a (more or less) gaussian mode. The amplifiers usually utilize crossed input and output polarizations in order to reduce input/output coupling and avoid oscillation.
Quasi-optical sources (oscillators) arranged in arrays have also been developed for millimeter wave power, and consist of a number of individual oscillators that are coupled together so that they mutually synchronize in phase and the radiation from all the elements combines coherently, typically in a (more or less) gaussian mode in front of the oscillator array. A number of different methods exist to realize the coupling network, from printed circuit transmission lines to partial reflectors. The key is to provide strong coupling between elements to ensure in-phase oscillation.
Many quasi-optical oscillator arrays utilize hardwire circuitry (e.g. printed circuits, waveguides) to couple together the oscillating elements. For these types of arrays it is very difficult to control or modify the coupling in real time, without resorting to complicated schemes that are difficult to realize. For quasi-optical arrays that utilize partial reflectors, the oscillators are usually one port devices (negative resistance oscillators) with a single polarization output, which increases parasitic mutual coupling, creating difficulty in controlling the coupling between elements.
The present disclosure takes quasi-optical arrays one step further by allowing the amplifier coupling to be easily controlled so that the array can be operated as an amplifier or as a coherent source, depending on the amount of array coupling set by the user.